The invention relates to the field of acceleration sensors. The invention is more particularly a slaved mobile plate accelerometer using capacitance variations to detect the movement of a mass.
Micromachined accelerometers are now well known in the sensor industry. They generally comprise a mobile plate, referred to as the seismic plate, suspended from a fixed armature by springs; the mobile plate/armature/spring assembly is obtained by chemically etching a silicon wafer.
The mobile plate comprises an electrode, for example. The fixed armature comprises two electrodes. The mobile electrode forms a capacitor with each of the two fixed electrodes. If acceleration is applied to the sensor, the mobile plate moves relative to the fixed armature, so creating an imbalance of the capacitances. A control circuit detects the difference between the two capacitances and reacts by applying a feedback voltage, and therefore an electrostatic force, between the mobile plate electrode and the fixed armature electrodes, in order to return the mobile plate to its original position. This provides an electrostatic feedback motor.
The electrostatic stiffness is known to be a function of the electrostatic force. For a sensor of the above kind, the principle of adjusting the electrostatic stiffness is as follows.
In the absence of electric field, a force Fm tends to return the mobile electrode to its rest position by virtue of the stiffness km of the springs (km is the derivative of the return force with respect to the displacement). However, in the presence of an electric field (voltage between the fixed electrodes and the mobile electrode), an electrostatic force Fe attracts the mobile electrode towards one or other of the fixed electrodes. This force is the sum of two forces in opposite directions proportional to the square of the electric field between the electrodes:
Fe=xcex50xc2x7(S1xc2x7E12xe2x88x92S2xc2x7E22), where Si is the surface area of the capacitor formed by the fixed electrode i and the mobile electrode, E=Vi/di, Vi is the voltage between the fixed plate electrode i and the electrode of the facing mobile plate, and di is the distance between the electrode i and the mobile plate electrode (i=1 or 2).
The electrostatic force is a function of the distance between the electrodes, and therefore of the displacement, like the spring return force, but in the opposite direction. Its derivative with respect to the displacement is the electrostatic stiffness ke.
It is also known that the frequency frm of mechanical resonance is related to the mass m of the mobile plate and to the stiffness km of the springs:       f    rm    =            1              2        ⁢        π              ⁢                            k          m                m            
The mass of the mobile plate is generally known accurately and closely controlled in manufacture, but this is not the case with the stiffness km of the springs. Performance therefore varies greatly from one manufacturing batch to another. This makes it necessary to solve a difficult problem in the manufacture of such accelerometers, namely that of obtaining a precise and reproducible frequency of mechanical resonance of the mobile plate/spring cell. The dynamic range (maximum signal-to-noise ratio) of this type of sensor is highly dependent on this frequency.
There is also another problem to be solved for accelerometers sensitive to the vertical component: sagging of the mobile mass due to the effect of gravity.
In vertical accelerometers (i.e. ones sensitive to the vertical component of acceleration), the mass sags due to its own weight by an amount xcex94z=mxc2x7g/km (m=mass of mobile plate, g=acceleration due to gravity, km=mechanical stiffness of springs).
Moreover, as the dynamic range of the accelerometer is proportional to the mass divided by the stiffness (S=m/k), a high mass and a low stiffness are required to achieve good performance. This leads to a large sag due to gravity.
In the vertical position, for a system with free deformation, the distance between the fixed plates and the mobile plates must be at least equal to the sag, as otherwise return systems must be used. However, too great a distance between the electrodes leads to poor performance because of problems with obtaining sufficient electric fields. The sagging therefore limits the performance of the sensor.
The mobile mass is generally centred on the armature in an attempt to solve these problems, which imposes the use of relatively complex manufacturing techniques such as:
1. prestressing the springs,
2. additional compensator springs,
3. electrostatic return (see U.S. Pat. No. 5,345,824),
4. remote electromagnetic return, and
5. additional fabrication steps (see U.S. Pat. No. 4,922,756).
U.S. Pat. No. 4,922,756 describes the fabrication of a micromachined sensor with the stiffness constants of the springs precisely controlled during the fabrication process. However, this is achieved at the price of additional fabrication technology steps.
U.S. Pat. No. 5,345,824 addresses the problem of the spread of the stiffness constant of the springs by minimizing the mechanical stiffness constant km but centring the mass by means of a small percentage of the total available electrostatic force. The output signal is therefore independent of the constant km because the springs are not deflected. However, this modifies the sensitivity of the accelerometer.
The skilled person therefore usually seeks a compromise between optimizing performance by reducing the stiffness constant, which increases the sag, optimizing performance by reducing the distance between the electrodes to obtain sufficiently high electric fields, which limits the possibilities of sagging, and optimizing the usable range of frequencies.
The device according to the invention is a slaved mobile plate accelerometer using variations of capacitance to detect the movement of a mass, it comprises:
at least one fixed electrode rigidly attached to an armature,
at least one mobile electrode suspended by springs from the armature and facing each fixed electrode to form at least one capacitor, each mobile electrode being adapted to move relative to each fixed electrode due to the effect of acceleration, so causing a variation of the capacitance of each capacitor, and
an electronic circuit for adjusting the electrostatic stiffness and comprising a control system for detecting the variation of the capacitance of each capacitor and reacting by applying a feedback voltage between each mobile electrode and the fixed electrode facing said mobile electrode,
characterized in that the springs have a stiffness chosen intentionally to place the mechanical resonant frequency beyond the top frequency of the band of interest and the circuit for adjusting the electrostatic stiffness is adapted to return the apparent resonant frequency into the band of interest.
A stiffness-adjusting device according to the invention of the above kind improves the performance of the system since it simultaneously:
1. compensates the spread of the mechanical stiffness of the springs suspending the mobile plate,
2. limits the sagging of vertical accelerometers by using a high mechanical stiffness, compensated by a high electrostatic stiffness, and
3. optimizes performance in the wanted band.
Note that deliberately placing the frequency of mechanical resonance beyond the top frequency of the wanted band overcomes the prejudices of the skilled person, this solution being at first sight unfavourable to the dynamic range, which is proportional to S=m/km.
In an advantageous embodiment of the accelerometer according to the invention, the accelerometer includes two fixed electrodes electrically insulated from each other.
In another advantageous embodiment of the accelerometer according to the invention the accelerometer includes a single mobile electrode.
In another advantageous embodiment, the electronic circuit of the accelerometer according to the invention enables time-division multiplexing of each mobile electrode.
In a further advantageous embodiment, the accelerometer according to the invention has a time-division multiplexing cycle which includes four steps:
a first step during which a voltage sample and its symmetrical counterpart relative to ground are respectively applied between each fixed electrode and the mobile electrode,
a second step during which the capacitor constituted by one of said two fixed electrodes and the mobile electrode and the capacitor constituted by the other fixed electrode and the mobile electrode are discharged,
a third step during which a feedback voltage is applied to one or the other of the capacitors constituted by the mobile electrode and one of the fixed electrodes, as a function of a decision taken by the control system, and
a fourth step during which the operation of the second step is repeated.
In another advantageous embodiment, the electronic circuit of the accelerometer according to the invention varies the amplitudes of the voltages between each fixed electrode and each mobile electrode to adjust the electrostatic stiffness.
A further advantageous embodiment of the electronic circuit of the accelerometer according to the invention enables the duration of the time-division multiplexing steps to be adjusted.
In a further advantageous embodiment of the accelerometer according to the invention, the duration of the time-division multiplexing steps is adjusted to adjust the electrostatic stiffness.
In a further advantageous embodiment of the accelerometer according to the invention, the electrostatic stiffness is adjusted to compensate the spread of the mechanical stiffness of the springs.
In another advantageous embodiment of the accelerometer according to the invention, the electrostatic stiffness is adjusted to reduce sagging when the accelerometer is in a vertical position, without loss of performance.
In another embodiment of the accelerometer according to the invention, the electrostatic stiffness is adjusted to optimize performance as a function of the wanted band.
The accelerometer according to the invention can also be a component of another, more complex device.